Recently, the conventional Public Switched Telephone Network (PSTN) is coexisting with the current IP network, the demand of which is rapidly increasing. Thus, the converged IP network is gaining attention as a next generation network since it is an IP network that can converge voice traffic, which is being serviced on the existing PSTN. The converged IP network converges different types of traffic such as data traffic, voice traffic and multimedia traffic on the IP network. In this case, the IP converged system processes the data, voice and multimedia traffics by converging these traffics on the IP network.
FIG. 1 is a configuration view illustrating the construction of an integrated IP network.
The integrated IP network includes a PSTN 110, an IP network 120 and IP converged systems 130a and 130b. 
The PSTN 110 routes calls to a number of subscribers by processing voice traffic.
The IP network 120 processes VoIP data traffic and common data traffic.
The IP converged systems 130a and 130b process both voice traffic and data traffic and can be simultaneously connected to both the PSTN 110 and the IP network 120. Examples of the IP converged systems 130a and 130b may include an IP-Private Branch exchange (IP-PBX), an Integrated Access Device (IAD), a home gateway, a Wireless Broadband Customer Premises Equipment (WiBro CPE) and so on.
Random Early Drop (RED) is a method of controlling network congestion, which randomly drops packets before congestion occurs. For this, the RED drops the packets by setting two thresholds including minimum and maximum thresholds to a queue and applying different packet drop probabilities to three sections. In detail, the RED operates as follows:
When an average queue size is smaller than the minimum threshold, all packets are allowed to pass through (No drop).
When the queue size is the same as or greater than the minimum threshold but smaller than the maximum threshold, packets are randomly dropped according to packet drop probabilities based on the queue size (Random drop).
When the queue size is the same as or greater than maximum threshold, all input packets are dropped (Tail drop).
As the number of packets stacked in the queue is increasing, the RED decreases the amount of incoming traffic by dropping more packets. When the maximum threshold is set too small, a severe effect on the entire performance can be caused due to frequent packet drops. Furthermore, an operation of dropping all incoming packets if the queue size is the same or greater than the maximum threshold is the same as the result caused by a buffer overflow. Hence, the RED generally sets the maximum threshold to be the same as or similar to the maximum size of the queue.
When packet drop probability is too low, packet drop frequency decreases and thus congestion control becomes difficult. Therefore, it is most important for the RED to properly set the minimum threshold, the maximum threshold and the packet drop probability.
However, since the RED randomly drops packets, even a high precedence packet can be dropped when the queue is full. The Weighted RED (WRED) was proposed to compensate for such drawbacks of the RED.
The WRED is designed to reduce the loss of important packets by weighting packet drop probabilities. Specifically, the WRED sets minimum and maximum thresholds and maximum drop probabilities to be different according to corresponding classes of traffics.
As described above, the IP converged systems 130a and 130b can be connected to both the PSTN 110 and the IP network 120. Thus, the IP converged systems 130a and 130b route an incoming call through the PSTN 110 or through the IP network 120.
Call routing in the network is enabled via the Least Cost Routing (LSR) so that least cost can be consumed. Generally, the IP network 120 is cheaper than the PSTN 110. Thus, the IP converged systems 130a and 130b generally route an incoming call through the IP network 120, but reroute the call through the PSTN 110 in an exceptional case.
The IP converged systems 130a and 130b reroute a call through the PSTN 110 based on a predetermined condition. The condition is, for example, the number of connecting VoIP calls and whether or not the link of the IP network 120 is down. For example, the condition of rerouting a call through the PSTN 110 may be the case when the number of connecting VoIP calls is one hundred (100). When one hundred (100) calls are being connected to the IP network 120, the IP converged systems 130a and 130b route other calls through the PSTN 120.
Conventionally, calls are rerouted through the PSTN when the link of the IP network is down or according to the number of concurrent calls. In this case, the IP converged systems 130a and 103b reroute a call irrespective of whether the processing state of common data traffic is idle or busy (congestion). In other words, the IP converged systems 130a and 130b reroute a call irrespective of the data traffic-processing state that has a direct effect on sound quality. The poor sound quality fails to ensure Quality of Service (QoS) and thus the IP converged system cannot ensure efficient use of resources.